check for layout files existence before writing

src/OcflStorageService.php

@@ -175,7 +175,9 @@ class OcflStorageService {
 
     // Create the OCFL root indicator file if it doesn't exist.
     $ocfl_root_indicator_path = $this->ocflRoot . '/.ocfl_layout';
+    if(!file_exists($ocfl_root_indicator_path)) {
       $this->fileSystem->saveData('ocfl_layout_1.0', $ocfl_root_indicator_path, FileExists::Error);
+    }
 
   }